Transmission control circuits in two-way signaling systems



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April 16, 1935. B. G, BJORNSON TRANSMISSION CONTROL CIRCUITS IN TWO-WAYSIGNALING SYSTEMS Filed Feb. 3, 1932 4 Sheets-Sheet 1 /NVE/VTOR BB.G.BJRNSON y A TTORNE V 'April 16, i935. B. G. BJoRNsoN "1,998,071

TRANSMISSION CONTROL CIRCUITSy IN TWOWAY SIGNALING SYSTEMSv Filed Feb.z, 1952. 4 sheets-sheet 2 JMW/gwn ATTORNEY pml I6, 1.9354 B. G. BJQRNSONTRANSMISSION CONTROL CIRCUITS IN TWO-WAY SIGNALING SYSTEMS Filed Feb. 5,1932 4 Sheets-Sheet A3 /VVE/VTOR B.G.BJONSON BV ATTOR/VE Y B; G.BJoRNsoN `TRANSMIVSSION CONTROL 4CIRCUITS IN TWO-WAY SIGNALING SYSTEMSApri; 1e, v193.5.

Filed' Feb. 5, 1932 /N VEA/TOR @y B. G. @JURA/.SON

VsmlAHHV ATTORNEY v @Nl UNITED STATES Partnr orties TRANSMISSION CONTROLCIRCUITS IN TWO-WAY SIGNALING SYSTEMS Bjorn G. Bjornson, Brooklyn, N.Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application February 3, 1932, Serial No.590,537

11 Claims.

This invention relates to two-way signaling systems and particularly tothe signal-controlled means employed therewith for suppressing echoesand preventing singing.

An object of the invention is to improve the operation characteristicsof long, two-way signaling systems employing signal-controlled apparatusfor suppressing echoes and preventing singing.

To obtain proper operation of a long two-Way signaling system, includingover at least a portion of its length a four-wire circuit having onepath adapted for transmission in one direction,

, and another path adapted for transmission in the ample, at a midwaypoint.

opposite direction, it has been found necessary in the past to employsignal-controlled apparatus for effectively disabling one ci theassociated one-way paths while signal transmission is taking place overthe other, so as to prevent echo currents or reected currents from beingtransmitted back to the transmitting end of the system and causing adisturbance or singing. The disabling apparatus usually comprises means,such as an amplier-rectier control circuit for diverting a portion ofthe signaling current from one path and utilizing it to control theopen-circuiting or short-circuiting of the oppositely directed path orto control in proper manner the operatic-n characteristics of anamplifier in the oppositely directed path. In certain circuits of theprior art, these signal-controlled devices or echo Suppressors as theyare commonly called, one of which is associated with each two-wire path,are located at some intermediate station, for eX- In other circuits oithe prior art, the tivo units of the echo suppressor are located at ornear the ends of the four- Wire circuit. The latter arrangement, thesccalled terminal echo suppressor, may be either of the sender or thereceiver type. In the sender type of terminal echo suppressor, thesuppressor units located near each terminal comprise a suppressorresponsive to signals in the outgoing two-wire circuit to disable theincoming two-wire circuit, and means responsive to signals in theincoming circuit to disable the suppressor. In the receiver type ofterminal echo suppressor, the suppressor units located near eachterminal of the four-wire circuit, comprise a suppressor responsive tosignals in the incoming two-wire circuit to disable the outgoingtwo-wire circuit, and means responsive to signals in the outgoingtwo-wire circuit to disable the suppressor.

The invention relates to improvements in twoway signaling systems, forexample, two-way telephone systems, employing echo Suppressors of theabove mentioned types, the improvements residing inthe provision ofmeans for facilitating break-ins by a listening party after one partyhas obtained exclusive control of the system. This is accomplished bythe provision of means under control of the rst talkers signalingcurrents operative a given time after he has Obtained control of thesystem, for rearranging the control circuits of the system so as to biasthe system from the standpoint of break-ins in favor of the listener.While the system is so biased, the listener can, on perceiving a pauseby the first talker, immediately break in, gain control of the systemfor his own signaling currents and shut out the i-lrst talker.

The nature of the circuits of the invention and their advantages will beclear from the iollowing detailed description thereof when read inconnection with the accompanying drawings:

Fig. l of which shows schematically a four-wire toll telephone circuitemploying terminal echo Suppressors of the receiver type and embodyingthe invention;

Fig. 2 of which shows schematically a four-wire toll telephone circuitemploying terminal echo Suppressors of the sender type, and embodyingthe invention;

Fig. 3 of which shows schematically a fourwire toll telephone circuitemploying echo suppressors of the mid-point type and embodying theinvention; and

Fig. 4 of which shows schematically a fourwire toll telephone circuitemploying the cornbination of receiver terminal with mid-point echoSuppressors and embodying the invention.

For convenience the invention will be described as applied to systemsfor the two-way transmission of speech waves, but it is to be understoodthat the principles of the invention apply equally well to systems forthe two-way transmission of signal waves in generalof frequencies withinor outside the speech frequency range, such as waves representingspeech, entertainment programs comprising music as well as speech, etc.

The four-wire toll telephone circuit of Fig, 1 comprises a one-waytransmission path EA including the one-way amplifying devices I and 2for repeating telephonic currents in the direction from west to eastbetween the west twoway telephone circuit L1 and the east two-wayVtelephone circuit L2, and the one-way transmission path WA including theone-way amplifying devices 3 and i for repeating telephonie currents inthe direction from east to west between the east two-way telephonecircuit L2 and the west two-way telephone circuit L1. In yell knownmanner, the oppositely directed one-way transmission paths EA and WA maybe connected in substantially conjugate relation with each other and inenergy transmitting relation with the telephone circuits L1 and L2,respectivel by the hybrid coil transformers H1 and I-I2, and theassociated balancing networks Ni and N2.

A receiver type terminal echo suppressor is located near the west end ofthe fourwire circuit, which will be referred to hereinafter' as stationA, and another receiver type terminal echo suppressor is located nearthe east terminal of the four-wire circuit, which will be referred tohereinafter as the station B.

The terminal echo suppressor located at station A includes a controlcircuit 5 having its input bridged across the output oi the path WA, andcomprising a relay-controlling device t, which may be a vacuum tubeamplier-rectier device such as is well known in the art, and thewindings of mechanical relays l, S and 9 connected in parallel to theoutput of the control device I5. The mechanical relay Q is made tooperate in response to operation oi the control device 5 a deniteinterval of time, :Z1 seconds, after the relays 'I and 8 have operated,for example, by the insertion of a delay circuit if: oi any suitabletype between the output of the device 6 and the winding of mechanicalrelay 5 designed to delay the transmission ci energizing current to thewinding of relay 9 for a time inter-- val of di.

The terminal echo suppressor at station A also includes a controlcircuit l I having,r its input connected across the path EA in theoutput of the amplifying device therein, and comprising arelay-controlling device I2, similar to the device 6, and the windingsof mechanical relays I3 and I4 connected in parallel to the output ofthe device I2. The mechanical relay I 4 is made to operate in responseto operation ci the control device I2 a denite time interval T after thedevice I2 has operated, for example, by the insertion of electricaldelay circuit I5, similar to the delay circuit I0, between the output oidevice I2 and the winding of mechanical relay I4, designed to delay thetransmission of energizing current to the winding of relay i4 for theinterval of time T, where T is the transmission time between station Aand station B over the transmission path EA.

In the input of the path EA at station A between the input of theamplifying device i and the hybrid coil H1 are the normally closedswitch contacts IG which are adapted to be opened by operation of themechanical relay 'l when its winding is energized by operation of thecontrol device 6 in control circuit 5. In the control circuit II betweenthe output of the delay circuit I5 therein and the winding of mechanicalrelay I4, are the normally closed switch contacts adapted to be openedby the relay 3 when its winding is energized by operation oi control device 6. In the output of the control device I2 in control circuit IIbetween the point of connection of delay circuit I5 thereto and thewinding of mechanical relay I3 are the normally open switch contacts I8adapted to be closed by the mechanical relay 9 when its winding isenergized by operation of the control device E.

In the path WA at a point therein at station A between the point ofconnection thereto of the control circuit 5 and the output of theamplifying device 4 are the normally closed switch contacts I 9 adaptedto be opened by relay I3 when its winding is energized in response tooperation o the control device I2 in control circuit 5 I. In the outputof the control circuit 5 between the output of the control device 6therein and the windings of mechanical relays 'I, and .i, are thenormally closed switch contacts 23 adapted to be opened by relay I4 whenits winding is energized in response to operation of control device i2in control circuit II.

The terminal echo suppressor at station B includes the control circuit2i having its input bridged across the output of the path EA, andcomprising a relay-controlling device 22, similar to the devices and i2at station A, and the windings of mechanical relays 23, 24 and 25connected in parallel to the output of the device 22. Relay 25 is madeto operate after the relays 23 and 24 in response to operation oi thedevice 22 by a time interval of di, for example, by the insertion of adelay circuit 26 between the output of the control device 22 and thewinding of relay 25 to delay the transmission of energizing current tothe winding of relay 25 for the interval di.

The terminal echo suppressor at station B also includes a. controlcircuit 27 having its input connected across the input of the path WA inthe output of the amplifying device 3 therein, and comprising a relaycontrolling device Zii, similar to the device 22, and the windings oimechanical relays 29 and 33 connected in parallel to the output of thedevice 23. The mechanical relay SI2 is made to operate after operationof the control device 28 by a time interval of preferably T, forexample, by the insertion of the delay circuit 3| in the control circuit2'1 between the winding or" relay 3S and the output of the controldevice 28, to delay the transmission of energizing current to relay 30from the output of the control device 23 for the time interval T.

In the input of the path WA at station B are the normally closed switchcontacts 22 adapted to be opened by relay 23 when its winding isenergized in response to operation of control device '.2 in controlcircuit 2|. In the control circuit 2'I between the output of the delaycircuit 3i and the Winding of relay 3i) are the normally closed switchcontacts 33 adapted to be opened by relay 24 when its winding isenergized by operation of the control device 22. In the output or" thecontrol circuit 2'5 between the point of connection thereto of delaycircuit SI and the winding of mechanical relay 29 are the normally openswitch contacts 34 adapted to be closed by relay 25 when its winding isenergized in response to operation of the control device E2 in thecontrol circuit 2 I. In the output of the control circuit 2i between theoutput of the control device Z2 therein and the windings of themechanical relays 23, 24 and 25 are the normally closed switch contacts35 adapted to be opened by relay 35i when its winding is energized inresponse to operation of the control device 28 in control circuit 2l.

The operation of the system of Fig. Il will now be described. It will beassumed that the circuit is inactive so that the switching contacts controlled by the echo suppressor circuits at both stations A and B are intheir normal condition as indicated in Fig. l. Then, let it be assumedthat a west subscriber associated with the twodyay circuit L1 starts totalk and that thereafter', but within a time interval of T, an eastsubscrice associated with the circuit L2 also starts to talk.

The west subscribers speech current received over the circuit L1 atstation A will be impressed by hybrid coil H1 upon the input of the westto east transmission path EA over which it will be transmitted afterampliiication by the amplifying device l at station A to the distantstation B where it will be amplified again by the amplifying device 2.The main portion of the west subscribers speech current in the output ofthe amplifying device 2 will be transmitted over the output of the pathEA to the hybrid coil H2 which will impress it upon the circuit L2 overwhich it will be transmitted to the listening east subscriber.

Meanwhile, a small portion of the west sub scribers speech current atstation A is diverted in the control circuit l and causes the operationof the control device l?- therein. The operation of control device i2,however, will not cause enerM gizing currents to be supplied to thewinding of the mechanical relay i3 because of the normally open switchcontacts i3 in the input of that relay. The operation of control devicel2 will cause energizing currents to be supplied to the winding ofmechanical relay Ill, but only after a delay of T due to the action orthe delay circuit l5. This means that the relay it will operatesubstantially at the same time as the west subscribers speech currentsarrive at the station B over the path EA, to open the normally closedswitch contacts 20 in the output of control circuit 5 to disable theinputs of mechanical relays ll, t and connected to the output of thecontrol device ii.

Thev portion of the west subscribers speech currents diverted intocontrol circuit 2l at station B will cause the operation of controldevice 22. The operation of control device 22 will cause energizingcurrents to be supplied directly to the windings of relays 23 and 24 andthrough delay circuit 26 to the winding of relay 25. Relay 23 willoperate immediately to open switch contacts 32 in the input of the pathWA rendering that path inoperative thereafter to transmit to the circuitL1 at station A or to the input of control circuits 2'! and 5. Relay 24will operate simultaneously with relay 23, to open the switch contacts33 in control circuit 21 thereby disabling the input of relay 3%. Thewest subscriber has now obtained exclusive control of the switchingcircuits at both station A and station B.

lf the east subscriber associated with circuit Lz starts to talkimmediately after the west subscriber does so, a portion of the formersspeech currents may pass the point in path WA where switch contacts 32are located before these contacts have been opened in response to thewest subscribers speech currents, and will cause operation of thecontrol device 23 in control circuit 2? at station B and of controldevice E in the control circuit at station A. A portion of the eastsubscribers speech currents will also be transmitted over the output ofthe path WA to circuit Li and will be heard by the west subscriberassociated therewith. The operation of control device will not causefalse operation of relays l, 8 and Si at station A because the input ofthese relays has been previously disabled by the opening of switchcontacts 2E! in response to the west subscribers speech currents.

The operation of the control device 28 will notJ cause energizingcurrents to be supplied to the winding of mechanical relay 29 becausethe switch contacts St in its input are still open. rEhe operation ofcontrol device 29, however, will cause energizing current to be suppliedtc the winding of mechanical relay 30 but with the' delay of T caused bythe delay circuit 3l in the output of the device 28. The transmission ofenergizing current to the winding of relay 3Q will be delayed for thetime interval T by delay circuit 3i and will therefore not be eiiectivein causing operation of that relay because its input will have beenpreviously disabled by the opening of switch contacts 33 in response tothe west subscribers speech currents. Then, the west subscribenhasobtained complete control of the switching circuits at both stations.

Substantially d1 seconds after the west subscriber has obtained controlof the switching circuits in the manner described, the energizingcurrents being transmitted through the delay circuit 25 in the output ofcontrol circuit 2i cause the winding of relay 25 to be energized. Relay25 then operates to close and maintain closed the normally open switchcontacts 3d in the output of control circuit 2l, thereby biasing thefourwire circuit as regards break-ins in favor of the east subscriber.Now, if the east subscriber breaks in during a pause by the westsubscriber sufficiently long to enable the relay 23 and the relay 2li torelease momentarily, his (the east subscribers) speech currents divertedinto control circuit at station A, will cause operation of the controlcircuit 2S therein to immediately energize the winding of relay 29causing the normally closed switch contacts 35 in the output of the pathEA to be opened to disable that path, and a time interval T later tovenergize the windof relay 3d causing the normally closed switch contacts35 in the output of control. circuit 2| to be opened to disable thatcircuit as regards subsequent operation under contro-l of the westsubscribers speech currents. A portion of the east subscribers amplifiedspeech currents transmitted over the path WA during this pause will bediverted into control circuit 5 at stationA and will cause operation ofthe control device 6 therein to energize the windings of mechanicalrelays l and 5 in the output thereof. These relays will immediatelyoperate respectively to disable path EA by opening switch contacts l5therein and to disable the input of relay lliby opening switch contactsil', thus giving complete control of the switching circuits to the eastsubscriber.

The operation of control device ii after a delay of an additionalinterval oi d; caused by delay circuit iii will cause the winding ofrelay 9 to be energized and that relay will operate to close normallyopen switch contacts i8 in the output of control circuit il, so as tobias the four-wire circuit as regards subsequent brealdns in favor ofthe west subscriber.

As indicated in Fig. l, the relay 9 in control circuit 5 at station Aand the corresponding relay 25 in control circuit 2l at station B shouldhave a small hang-over, h1, in their operation, and the relay i3 incontrol circuit i i at station A and the relay 2S in control circuit 2lat station B should have a small hang-over, hz, in their operation.These hang-over times should be such as to kee relays i3 and 29 operatedto maintain the switch contacts i9 in the path WA and the switchcontacts 36 in the path EA open, respectively, until the dissipation ofthe energy which might have passed by the controlling suppressor beforeit came into operation. These hang-overs may be F obtained by propermechanical design of the relays to make them slowreleasing, or by utiiizing in connection t erewith auxiliary hang-over circuits, for example,such as disclosed in the patent to J. Herman, 1,798,202, issued March3l, 1931. The echo suppressor relay 'l at station A and the echosuppressor relay 23 at station B should also have suicient hangover intheir operation to hold them operated for a slight time aiter the supplyof speech currents to the control devices 6 and 22, respectively, ceasesto prevent premature closing of the switch contacts l5 and 32,respectively.

Fig. 2 shows a circuit arrangement for facilitating bres-.loins by alistening party after one party has obtained exclusive control in afourwire toll telephone circuit employing sender terminal echoSuppressors.

The echo suppressor circuits at station A in the system of 2 includes acontrol circuit 3l' having its input connected across the path EA atstation A, and comprising a relay controlling device which may be avacuum tube amplifier-rectifier circuit, the winding of a mechanicalrelay il connected to the output or" the control device 33 over acircuit 4l, and the windings of mechanical relays l2 and 43 connected tobe operated in parallel from the output oi the control device 33 over acircuit 4@ including the delay circuits fl5 and 5S. Connected around theportion ci control circuit @Il includingr the delay circuit d5 of relay43 is a by-pass circuit The normally closed switch contacts d8 in thepath WA at station A are adapted to be opened to open-circuit the outputol the path WA by means of relay in response tc energization ci itswinding by the operation of control device 33. The normally closedswitch contacts le in the path WA at station A are also adapted to beopened to open-circuit the output of path WA by relay i? when itswinding is energized by operation of the control device 38. The normallyclosed switch contacts 5B in by-pass circuit 4l are adapted to be openedto disable that circuit by relay 43 when its winding is energized byoperation of the control device 38.

Bridged across the output of the path WA between the point of connectiontherein of switch contacts 45 and 9 and the hybrid coil H1 is the inputof a control circuit 5| comprising the control device 52, similar to thedevice 38, and the windings of mechanical delays 53, 54, 55 and 56connected in parallel with the output of the control device 52. Theoperation of relay 55 is delayed with respect to that of relays 53, 54`and 55 in response to the operation of control device 52 for a smalltime interval di by a delay circuit of suitable design connected betweenthe winding of relay 55 and the output of the control device 52.

The normally closed contacts 58 in the input of the control circuit 3'!are adapted to be opened to disable that control circuit by relay 53when its winding is energized in response to operation ci the controldevice 52. The normally closed switch contacts 5S in the output of thecontrol circuit 114 are adapted to be opened to disable the input ofrelay 42, by relay 55 when its winding is energized in response tooperation of the control device 52. The normally closed switch contacts55 in circuit 44 between delay circuit 45 and delay circuit 48 areadapted to be opened to disable the circuit 44, and therefore relays 42and 43, by relay 54 when its winding is energized in response tooperation of the control device 52. The normally open switch contacts 6lin the circuit 4I connecting the output of the control device 38 to thewinding of relay 4D, are adapted to be closed to render operative thecircuit 4l, by relay 56 when its winding is energized by operation ofthe control device 52.

The echo suppressor circuits at station B in the system of Fig. 2include a control circuit 62 having its input connected across the inputof the path WA at station B, and comprising the relay controlling device63, similar to the devices 52 and 33 at station A, the winding of amechanical relay 64 connected to the output of the device 53 by acircuit 65, and the windings of mechanical relays 66 and 6l connected tobe operated in parallel from the output oi the control device 53 over acircuit 68 comprising the delay circuits 69 and 75.

Connected around the portion of the circuit 68 including the delaycircuit 'lo and relay 61 is the by-pass circuit ll.

rIhe normally closed switch contacts l?. in the output of the path EAare adapted to be opened to open-circuit that path by relay Eid when itswinding is energized in response to operation of the control device 63.The normally closed switch contacts 'i3 in the output of the path EA areadapted to be opened also to open-circuit that path by relay 56 when itswinding is energized in response to operation of the control device 53.The normally closed switch contacts 'M in the by-pass circuit 'H areadapted to be opened to disable that circuit by relay 6l when itswinding is energized in response to operation oi the control device 63.

Connected across the output of the path EA between the switch contacts'l2 and 'I3 therein and the hybrid coil H2 is the input of a controlcircuit 'l5 comprising the relay-controlling device i6, similar to thedevice 63, and the windings of mechanical relays l1, 18, 18 andconnected in parallel across the output of the control device l'. Therelay 8i) is made to operate a small time interval, d1, after relays ll,'.'8 and 19 have operated in response to operation of the control device16 by means of a dela-y circuit 8i of suitable design connected betweenthe winding in response to operation of the control device '56.

The normal closed switch contacts 82 in the input of the control circuit62 are adapted to be opened to disable that circuit by operation ofrelay TI when its winding is energized in response to operation of thecontrol device 76. The normally closed switch contacts 83 in the outputof the circuit S8 are adapted to be opened to disable relay 66, by relay'I9 when its winding is energized in response to operation of controldevice 16. The normally closed switch contacts 84 in the circuit 68between delay circuits G9 and 'ID are adapted to be opened to disablethat circuit, by relay I8 when its winding is energized in response tooperation of control device l5. The normally open switch contacts 85 inthe circuit 65 are adapted to be closed to render operative the circuit65 connecting the winding of relay 64 to the output of the controldevice 63, by relay 80 when its winding is energized in response toloperation of the control device 16.

The delay circuits 45 and 46 in the circuit at station A, and the delaycircuits 65 and 'lil in the circuit 68 at station B are each designed toproduce a delay T in the transmission of currents therethrough, Where'I' is the transmission time between the stations A and B over one ofthe one-way transmission paths EA or WA.

The operation of the system of Fig. 2 will now be described. rIt Will beassumed that the west subscriber associated with circuit L1 terminatingat station A starts to talk before the east subscriber associated withcircuit L2 terminating at station B starts to talk. The west subscribersspeech currents will be impressed by the hybrid coil H1 upon the inputor" the path EA and transmitted thereover towards station B. A portionof the 'west subscribers speech currents will be diverted into the inputof the control circuit 3l causing the operation of the control device 38therein.

The main portion of the west subscribers speech currents will betransmitted out over the path EA to the distant station B being ampliedin transit by the amplifying devicesY in that path. At station B, themain portion of the west subscribers amplified speech currents will betransmitted over the path EA to the hybrid coil H2 which will impress itupon the circuit L2 over which it will be transmitted to the listeningeast subscriber'.

A small portion of the west subscribers'speech currents at station Bwill be diverted into the control circuit 'l causing the operation ofthe control device lt therein. The operation of the control device iiiwill cause energizing current to be supplied to the windings ofmechanical relays ll to dt. Relay "all will operate immediately to openthe normally closed switch contact-s "2 in the input of the controlcircuit S2 making that circuit thereafter inoperative to transmit speechcurrents from the east subscriber thereafter received from the path WA.Relay i3 will operate simultaneously with relay Tl to open the normallyclosed switch contacts Sd in the output of the delay circuit ESQ incircuit G3 thereby preventing any currents which may be stored in thedelay circuit SS before the switch contacts S2 in the input of controlcircuit E2 have opened from being transmitted to the delay circuit llior to the circuit "l, The relay i9 will operate simultaneously withrelays 'il and 'i3 to open the normally closed switch contacts 33 in theinput of the relay 5S' thereby preventing false operation thereafter ofthat relay by speech currents which may have been stored in the delaycircuit it at the time relay 'il operates to disable the input of thecontrol circuit 62. Meanwhile at station A the operation of the controldevice t8 under control of the west subscribers speech currents hascaused energizing currents to be transmitted through delay circuit i5 incircuit which will be divided between the 1oy-pass circuit and the inputof the delay circuit it in circuit dfi. The portion transmitted throughthe by-pass circuit di will cause the winding ofY relay to be energized,and the relay l2 will then operate immediately to disable the path WA atstation A by opening the normally closed switch contacts t9 in thatpath. Because of the delay of T introduced by delay circuit 55, therelay @i2 will operate to, disable the path WA by opening switchcontacts i9 therein substantially at the same time as the relays 'El' toEQ have operated at station E to disable the control circuits thereatwhich normally would be responsive to the east subscribers speechcurrents. The west subscribers speech currents now have obtainedcomplete control of the switching circuits at both terminal stations. Y

if the east subscriber associated with 'the cir-` cuit Lz starts to talkimmediately after the west subscriber associated with the circuit L1, itwill be apparent that with the circuit in the operated conditiondescribed, the east subscriber will not be able to get his speechcurrents through to the west subscriber or to seize control of theswitching circuits away from the west subscriber because the switchcontacts in their path will have been previously opened in response tothe west subscribers speech currents before the east subscriberscurrents have reached the switching points.

Now, the portion ofthe energizing currents transmitted to the input ofdelay circuit in the circuit di will be delayed in transmissiontherethrough another interval4I of T after which it will be supplied torelays 42 and d3. Relay 43 will then o-'ierate to Vopen the normallyclosed switch contacts te in the by-pass circuit 41 thus cutting off thesupply of current to the relay-42 over that circuit. However, relay 42'will not release because it will be maintained energized by the currentsfrom the output of delaycircuit d6 transmitted through unilateral deviceil to the winding of that relay.

Substantially di seconds after relays 'i1 to 'i9 have operated atstation B, relay 86, the energization of the winding of which has beendelayed or that interval of time due to the action of delay circuit ti,will operate to close the normally open switch contacts S5 in thecircuit 65 at station B rendering that circuit operative thereafter tosupply energizing current to the winding of relay @il immediately onoperation of the control device 63 in the path 62. Then, the eastsubscriber on perceiving a pause in the continuous transmission of thewest subscribers speech currents sufficiently long to cause relays 'iland 2 at stations B and A, respectively, to release, may break in andseize exclusive control of the switching circuits at both terminalstations during the pause in the following manner.

IThe east subscribers speech currents will be impressed upon the inputof the path WA by hybrid coil H2. The main portion will be transmittedthereover to the distant station A, and a small portion will be divertedinto control circuit t2, the switch contacts 82 therein being in theclosed condition during the pause and will cause the operation of thecontrol device t3. The opration of the control device 63 will causeenergizing currents to be transmitted both to the circuit 68 and, theswitch contacts 85 still being in the closed condition due to thehangover of h1 in the operation of relay 8@ controlled by the westsubscribers speech currents, also to the circuit S5. The energizingcurrent impressed upon the circuit will be immediately transmittedthereover to energize the winding of control relay 64 whichwill'immediately operate to open the normally closed switch contacts 'l2in the output of the path EA making that path thereafter inoperative totransmit the west subscribers speech currents to the circuit L2. Theenergizingy currents transmitted to the circuit 68 from the output ofcontrol device dit will be delayed in that circuit in transmissionthrough delay circuit 69 for an interval or" T after which they will bedivided between the by-pass circuit 1I and the delay circuit l. vTheportion of the currents transmitted over the by-pass circuit Tl willcause energizing currents to be transmitted to the winding of relay 66causing that relay to operate immediately to put an additional opencircuit in the output of the path WA by opening normally closed switchcontacts '13. The currents transmitted to delay circuit will be delayedtherein for a time interval of T and subsequently will cause operationof relay 67 tov disable the by-pass circuit 'll and through uni-lateraldevice 58. will maintain thel relay 5S operated for the additional timeinterval T in a manner similar to that explained above for thecorresponding apparatus at station A. n

Meanwhile, the east subscribers speech currents which have beentransmitted over the path WA will arrive at station A. The main portionof these currents will be transmitted from the output of the path WA tothe hybrid coil H1 which will impress it on circuit L1 over which itwill be transmitted to the listening west subscriber. A portion of theeast subscribers speech currents in the path WA at station A will bediverted into the control circuit 5i and cause the operation of thecontrol device 52 therein to supply energizing currents to the windingsof relays 53 to 56 ccnnected to its output. Relays 53, 54 and 55 willoperate simultaneously to open switch contacts 58 in the input or"control circuit 31, switch contacts 5t the input of control relay 42 andswitch contacts SS in circuit 45 to seize control of the switchingcircuits at the station A for the east subscriber. Because of the delayof T introduced by delay circuit S9 in the circuit 68 at station B, itwill be seen that the control relay 66 at that station operates to seizecontrol of the switching circuits thereat for the east subscribersubstantially at the same time as the relays 53, 54 and 55 at station Aoperate to seize control of the switching circuits thereat for the eastsubscriber.

A period of time di after relays 53 to 55 at station A have operated,the energizing currents from the output of control device 52, which havebeen delayed in delay circuit 5l, will cause the Winding of relay 56 tobe energized so as to cause operation of that relay to close normallyopen switch contacts 5l in the circuit 56 biasing the system as regardssubsequent operation of the switching control in favor o the westsubscriber.

Proper operation or" the system of Fig. 2 requires that the relays 54and 55 controlled by the control device 52 at station A, and thecorresponding relays 'I8 and lil controlled by the control device-16 atstation B have a hang-over in their operation of substantially T, whereT is the transmission time between station A and station B over oneone-Way path therebetween. As indicated in the drawings, the relay 5Scontrolled by the control device 52 at station A and the correspondingrelay 8S controlled by the control device 16 at station B should bedesigned to have a smaller hang-over in operation of h1, and the relay45 controlled by the control device 38 at station A` and thecorresponding relay 64 controlled by the control device 63 at station Bshould have a smaller hang-over of h2 in their operation. The hang-overtimes h1 and h2 should be sufllciently long to keep relay 40 and relay64 operated until the dissipation of the energy which may have passed bythe input of the controlling suppressor before it came into operation.The suppressor relay 42 at station A and the suppressor relay 66 atstation B also should be designed to have suicient hang-over times intheir operation to prevent premature closing of the switch contacts 4Sin path WA and switch contacts 13 in the path EA with cessation ofsupply of speech currents to the control `device 38 and the controldevice B3, respectively. Thehang-overs in the several relays may beobtained by suitable mechanical design of these relays to make themslowreleasing to the required degree or by employing in connection,therewith suitable auxiliary hangover circuits. The relay 53 controlledby the control device 52 at station A and the relay 'il controlled bythe control device 'l5 at station B are preferably designed to havelittle or no over in their operation so that they will releasesubstantially instantaneously with cessation in the supply of voicecurrents to the controllingdevices.

Fig. 3 shows a circuit arrangement for facilitating break-ins by alistening party after another party has obtained exclusive control in afour-wire toll telephone circuit employing echo Suppressors of themid-point type.

As indicated in Fig. 3the echo suppressor circuit located at some pointintermediate the two terminals of the four-wire circuits, which Will bereferred to hereinafter as station M, includes a control circuit 86having its input Yfmnnected across the path EA at station M, andcomprising a relay-controlling device 81, which may be a vacuum tubeamplier-rectier circuit wel] chown in the art.

A mechanical relay 8E is connected to the output of the control device8l by a circuit 3:9. A mechanical relay 9B is connected to the output ofthe control device 8'! by a circuit 9G. A niechanical relay S2 isconnected to the output of control device 8l' by a circuit 93 includinga delay circuit 34. Two mechanical relays e5 and are connected acrossthe circuit S3 in the output or" the delay circuit Si! by a circuit 9'!including a delay circuit 98.

Connected across the path WA at station M is the input of a controlcircuit 9S comprising a relay-controlling device lo@ similar to thedevice 8l. A mechanical relay lill is connected to the output of thedevice H30 by the circuit m2. A mechanical relay 193 is connected to theoutput of the device Il by the circuit m4. A mechanical relay |85 isconnected to the output of the device 100 by the circuit |06 includingthe delay circuit It?. The two mechanical relays Hi8 and H353 areconnected in parallel across the circuit |55 in the output f the delaycircuit I6? by a circuit l!) including the delay circuit Ill.

The normally closed switch contacts H2 in the path WA at station M areadapted to be opened to disable that path by relay 88 when its windingis energized by operation of control device l. The normally closedswitch contacts i i3 in the output of circuit H15 are adapted to beopened to disable the input of relay 1&5 by relay 9) when its winding isenergized by operation of the control device 8l. The normally cle-sedswitch contacts l I4 in the path WA are adapted to be opened to disablethat path by relay 32 when its Winding is energized by operation oicontrol device 31. The normally closed switch contacts l l5 in circuitE39 are adapted to be opened to disable the input of relay 8S by relay95 when its Winding is energized by operation of the control device 81.The normally open switch contacts H5 in circuit 104 are adapted to beclosed to render the input of relay 33 operative by relay 9E when itswinding is energized by operation of control device 8l.

The normally closed switch contacts lil in the path EA at station M areadapted to be opened to disable that path by relay icl when its windingis energized by operation of control device |00. The normally closedswitch contacts H8 in the output of circuit 93 are adapted to be openedto disable the input of relay 92 by relay I B3 when its Winding isenergized by operation of the control device I 0D. The normally closedswitch contacts ils in the path EA at station M are adapted to be openedto disable that path by relay iii when its winding is energized byoperation of control device itil. The normally open contacts l2@ incircuit Si are adapted to be closed to render operative the input oirelay QS by relay when its winding is energized by operation oi thecontrol device Idil. The normally closed switch contacts l2! in circuitE92 are adapted to be opened to disable the input of relay iti by relayit when its winding is enk ergized by operation of the control deviceMIIB.

The operation of the system oi' Fig. 3 will now be described. It will beassumed that the west subscriber associated with the circuit L1 startsto talk irst.

The west subscribers speech currents will be impressed upon the input ofthe path EA by hybrid coil H1 at station A and after being amplied bythe amplifier at that station will be transmitted over the path EAtowards the distant station B. The main portion or" the west subscribersspeech currents, after amplification by the various amplifiers in itspath, will be transmitted out over the output of the path EA to thehybrid coil i-lz which will impress it on the circuit L2 over which itwill be transmitted to the listening east subscriber.

At the intermediate station M a small portion of the west subscribersspeech currents is diverted into the control circuit 36 and will causethe operation of the control device 3l therein. The operation of thecontrol device 8l will supply energizing current to the relay 83 overthe circuit S9 and will also supply energizing current to relay Q2 overthe circuit t3 through the delay circuit gli, and will supply energizingcurrent to the. windings oi relays d5 and 95 over circuit 93 throughdelay circuit Sii and over circuit di through delay circuit 9,3.

Relay 38 will immediately operate whereas the operation of relay Si?will be delayed for a time interval of T where T is the transmissiontime between the station A and station B over the path EA, and relaysand 9S will operate after a delay of T-l-di, where di is any desiredtime interval.

The operation of relay St will open the normally closed switch contactsH2 in the path WA at station `M rendering that path inoperativethereafter to transmit the east subscribers speech currents to the westsubscriber associated with the circuit Li or to the control circuit 99at station M.

After an additional time interval T provided by the delay circuit Sie inthe circuit 93, relay 92 will operate to further disable the path WA atstation lvl by opening the normally closed switch contacts iid therein.An interval of time d1 seconds later required for the energizingcurrents to be transmitted through delay circuit 98, the relays @E andil?) will operate simultaneously. The operation or relay 25 will opennormally closed switch contacts i l5 in circuit 859 thereby disablingthe input of relay which will release after a hang-over interval or h2causing contacts H2 to return to their normal closed condition. Relay @twill simultaneously operate to close normally open switch contacts l lbin the circuit 64, making that circuit operative thereafter to transmitenergizing currents from the control device 50 to the winding of relayifwhen the control device i353 is operated in response to currentsimpressed upon the input of control circuit 99.

From the above description, it is apparent that if the east subscriberassociated with the circuit Lz terminating at station B starts to talkafter the west subscriber but within the time interval of T, the formerwill get his speech currents started within the time T through to thewest subscriber associated with the circuit L1 over the path WA. Inother words, the west subscriber can hear what the east subscriber saysbefore the end of the time interval T. However the east` subscribersspeech current transmitted over the path WA within the time interval T,although it will cause operation of the control device itil in controlcircuit at station M, will not be able immediately to seize control ofthe switching circuits at station M away from thewest subscriber becauseof the open switch contacts in the input of relay 93, and the delaycircuits lill and Hl in circuits IFBB and HB, respectively, providing adelay of T and oi T-l-di in the transmission of energizing current torelays H35, IES and |69, respectively.

However, at the end oi the interval T+d1. relay Sii has operated torender circuit |94 operative and relay Q5 has operated to disable thecircuit Se leading to the winding of relay 8, and the echo suppressorcircuits are now biased in favor of the listening east subscriberassociated with the circuit Lz terminating at station B, and he (theeast subscriber) on perceiving a pause in the west subscribers speechtransmission may break in and seize control of the switching circuitsaway from the west subscriber in the following manner.

The relays 95 and 9E have a small hang-over of 7u seconds during whichthey will remain operated after the west subscriber stops talking. Now,if the east subscriber says something when he observes west pause, his(easts) speech currents will be impressed upon the path WA by hybridcoil H2 at Station B and will be transmitted over that path to station Marriving thereat at the time at which relay 92 has released during thissame pause to return the switch contacts l lli in path WA to theirnormally closed condition.

A portion of the east subscribers speech currents, thereiore, will bediverted into control circuit Se causing the operation oi control deviceISS therein. The operation of the device ISG, the circuit llill beingstill in the operative condition in which the switch contacts l I6 areclosed because of the hang-over of h1 in the operation oi relay Sii,will energize the winding oiy relay I causing that relay to open thenormally closed switch contacts H8 in circuit 93 to prevent the westsubscriber from regaining control of relay 92 at the end of the pause inhis conversation.

At the end of the time interval T after control device lili! has beenoperated by the east subscribers speech currents, the energizingcurrents which have been delayed in transmission through delay circuitlul, will be transmitted to the winding of relay i535 causing theoperation oi that relay to open normally closed switch contacts l i9 inthe path WA at station M, rendering that path inoperative thereafter totransmit the west subscribers speech currents to the control circuit 86or to the east subscriber associated with circuit L2 over the output ofthe path EA. The east subscriber has now obtained exclusive control ofthe switching circuits of the system.

At the end of an additional interval of timey of d1 the energizingcurrents, which have been delayed in transmission through delay circuitHl, causethe operation of relays 108 and |09 to render operative thecircuit 9| by closing switch contacts |2|| therein, and to renderinoperative circuit |32 by opening normally closed switch contacts |2|therein. The system is now biased as regards break-ins in favor of thewest subscriber and, on perceiving a pause by the east subscribersufficient to cause relays |35 and mi to release momentarily, he may nowbreak in and seize control of the system away from the east subscriberin a manner similar to that de scribed in connection with the conversecondition.

The relays |68 and |39 controlled by the control device |38 have a smallhang-over of h1, equivalent to that of relays and 98 controlled bycontrol device 8l and provided for a similar purpose. Relay |0|controlled by the control device |63 and relay 83 controlled by thecontrol device 31 each have a small hang-over in their operation of h2,r1"his hang-over is sufficient to insure that these relays will notrelease during the pause in the speech currents of the subscriber whohas controlled their operation, to unblock the path over which thecurrents of the subscriber attempting to break in are being transmittedat the time these currents arrive at the blocking point in that path.These hang-over times may be obtained in well known manner by design oithe several relays to make them slow-releasing to the required degree orby en-- ploying in connection with these clays auxiliary hang-overcircuits.

Fig. 4 shows a circuit arrangement for facilitating break-ins by alistening party when one party has obtained control in a fourwire tolltelephone circuit employing the receiver type oi echo Suppressors at thetwo terminal stations A and B and mid-point type Suppressors at anintermediate station M.

The suppressor circuits at station A include a control circuit |2|having its input connected across the output of the path WA at stationA, and comprising a relay-controlling device |22, such as a vacuum tubeamplier-rectier circuit, and the windings of mechanical relays |23 and|24 connected in parallel to the output of the device |22. A delaycircuit |25 for introducing a delay of d1 seconds in the transmission ofenergizing currents to relay |23 when control device |22 is operated, isinserted between the output oi the devic-e |22 and the winding of relay|24, to insure that relay |24 operates a denite time after relay |23.The echo suppressor circuits also include the control circuit |26 havingits input connected across the input of the path EA at station A, andcomprising a relay-controlling device |27, similar to the device |22,and the winding of a mechanical relay |28 connected to the output of thedevice |21. The normally closed switch contacts |29 in the input of thepath EA between the point oi connection of control circuit |25 theretoand a hybrid coil H1, are adapted to be opened by relay |23 when itswinding is energized by operation of the control device |22. Thenormally open switch contacts |33 in the output oi control circuit |25are adapted to be closed to render operative the input of relay |23, byrelay |24 when its winding is energized in response to operation of thecontrol device |22. The normally closed switch contacts |3| in theoutput of the path WA at station A east of the point of connection ofcontrol circuit |2| to the path WA, are adapted to be opened to disablethe path WA, by

relay |28 when its winding is energized in response to operation ofcontrol device |127.

The echo suppressor circuits at station B include a control circuit |32having its input connected across the output of the path EA at stationB, and comprising a relay-controlling device |33 similar to the devices|22 and |27, and the windings of mechanical relays |34 and |33 connectedin parallel to the output of the device ifi-5. A delay circuit |33connected between the output of the control device |33 and the windingof relay |35 is provided to produce a time delay oi d1 in the operationof relay |35 with respect to the operation of relay |34 in response tothe operation of control device |33.

The echo suppressor circuits at station B also include a control circuit|31 having its input connected across the input of the path WA atstation B, and comprising a relay-controlling device |38, similar to thedevice |33, and the winding of a mechanical relay |39 connected to theoutput or" the device |38.

The normally closed switch contacts |40 in the path WA between the pointof connection thereto of control. circuit |31 and the hybrid coil H2,are adapted to be openedto disable the input of the path WA, by relay|34 when its winding is energized by operation of the control device|33. The normally open switch contacts |4| in the output of the controlcircuit |37 are adapted to be closed to render operative the input ofrelay |39, by relay |35 when its winding is energized by operation ofcontrol device |33. The normally ciosed switch contacts |42 in the pathEA at station B to the west of the point of connection of the controlcircuit |32 to that path, are adapt-ed to be opened to disable theoutput of the path EA, by relay |39 when its winding is energized inresponse to operation of the control device |38.

The echo suppressor circuits at station M include a control circuit |43having its input connected across the path WA at station M, and cmprising a relay-controlling device |44, similar to the device |33, andthe windings of mechanical relays |45 and |45 connected in parallel tothe output of the device |44. A delay circuit |47 providing a delay of Tin the transmission of current therethrough, where T is the transmissiontime between station A and station B over the path EA or the path WA, isinserted between the winding of relay |48 and the output of controldevice MA, to delay the operation of relay |46 with respect to that of|45 by the time interval T in respense to operation of control device|34.

The normally closed switch contacts |48 in the path EA at station M areadapted to be opened to disable that path, by relay |45 when its windingis energized in response to operation or" control device |44. Thenormally closed switch contacts |49 in the control circuit |43 areadapted to be opened to disable the input of relay |'|5, by relay |46when its Winding is energized in response to operation of the controldevice |44.

The echo suppressor circuits at station M include a control-circuit |53`having its input connected across thel path EA at station M, andcornprising a relay-controlling device |5|, similarV to the device |44,and the windings of mechanical relays |52 and |53 connected in parallelto the output of the device |5|.

A delay circuit |43 is connected between the winding |53 Vand the outputof the control device |5| to provide'a delay of Tin theoperation ofrelay |53 with respect to that of relay |52 in response to the operationoi control device l5 l.

The normallyl closed switch contacts |55 in the path WA at station M areadapted to be opened to disable the path WA, by relay |52 when itswinding is energized by operation of control device |5|. The normallyclosed switch contacts 55 in the output of control circuit |53 areadapted to be opened to disable the input of relay |52, by relay |53when its winding is energized by operation of control device |5i.

The relays |45, m3, |52 and |53 in echo suppressor circuits at station Mare designed to have a hang-over of T in their operation. The relay |24in control circuit |2| at station A and the corresponding relay |35 inthe control circuit |32 at station B are designed to have a smallerhangover in their operation of h1, soA as to maintain switch contacts|33 in control circuit |26 at station A and switch contacts Ml incontrolcircuit |37 at station B, respectively, closed for a sufficienttime interval after the control devices |22 and |33, respectively,release during a pause in the continuous transmission of the controllingspeech currents of the talking subscriber to enable the subscribertrying to break in to seize control of the switchingA circuits duringthat pause. The relay |23 in control circuit |26 at station A and therelay |39 in control circuit |31 at station B are designed to have ahang-over in their operation of substantially T. These hang-over timesmay be provided by suitable mechanical design of the various relays tomake them slowreleasing to the required degree, or by employing inconnection with these relays suitable auX- iliary hang-over circuits.

The operation of the system of Fig. 4 will now be described. It will beassumed that the west subscriber associated with the circuit L1terminating at station A first starts to talk. His speech currents willbe impressed by the hybrid coil H1 upon the input of the path EA andtransmitted thereover towards the distant station B.

The main portion of the west subscribers speech currents will betransmitted over the path EA being ampliiied en route by the variousamplifying devices in that path, to the hybrid coil H2 at station Bwhich will impress it upon the circuit L2 over which it will betransmitted to the listening east subscriber. A small portion of thewest subscribers speech currents, however, will be diverted into thecontrol circuit |25 at station A, causing the operation of the controldevice |21 therein. The operation of the control device |21, however,will not cause operation of the relay |28 in its output because of thenormally closed switch contacts |36 in the input of that relay.

Another portion of the west subscribers speech currents will be divertedinto the control circuit |53 at station M causing the operation ofcontrol 'device |5| therein. The operation of control device l5! willcause energizing currents to be immediately supplied to the windingofrelayr |52 causing immediate operation of that relay to disable the pathWA at station M by opening the normally closed contacts |55 in thatpath. Substantially T seconds later the winding of relay |53 will beenergized. by the currents from the output or the control device |54which have been delayed in delay circuit |54, causing the operation ofthat relay to open the normally closed switch contacts |53 in the inputof relay |52. The relay 52, because of the hang-over in its operation,however, will not release for an additional interval or time Tthereafter, at the end of which interval it will cause the switchcontacts |55 to return to their normally closed condition rendering thepath WA operative thereafter for transmitting current thereover.

Another portion of the west subscribers speech currents will be divertedinto the control circuit |32 at station B causing the operation ofcontrol device |33 therein. The operation of control device !33 willimmediately supply energizing currents to relay |34 causing itsimmediate operation to open-circuit the input of path WA at station B byopening the normally closed switch contacts |43 in that path.

The west subscribers speech currents have now obtained exclusive controlof all the switching circuits in the system. p

Substantially d1 seconds after the west subscribers speech currents haveobtained exclusive control of the system, the system is biased asregards break-ins in favor of the east subscriber. under control of thewest subscribers speech currents in the following manner.

Substantially d1 seconds after relay |34 has operated in response tooperation of control device |33, the energizing currents for relay |35which have been delayed for that interval of time of delay circuit |36are supplied to the winding of relay |35 causing its operation to closenormally open switch contacts |45 in control circuit |31, thereafterrendering that circuit operative to transmit energizing currents torelay |39 from the output or control device |38 when that olevice isoperated. Relay |35 because of the hangover in its operation will remainoperated h1 seconds after the west subscriber has stopped talking.

Now, let us suppose that the east subscriber associated with the circuitL2 terminating at station B says something when he observes the westsubscriber pause. The east subscribers speech currents will be impressedby the hybrid coil H2 on the input of the path WA and, the relay |36having released during this pause by the west subscriber to temporarilyreturn switch contacts |40 to their normally closed condition. eastsspeech currents will be transmitted over the path EA towards the weststation A. A portion of the east speech currents will be diverted intocontrol circuit |31 at station B causing the operation of control device|38 therein. Because of the hangover of 71,1 in the operation of relay|35 the switch contacts IM in the output of control circuit |31 arestill in the closed conditions() that energizing currents are suppliedto the winding of relay |39 which operates to open the normally closedswitch contacts |42 in the output of the path EA at station B renderingthat path thereafter inoperative to transmit to circuit Le or to thecontrol circuit |32. Control circuit |32 having been thus disabledduring the pause in the west subscribers speech currents, the westsubscriber will not be able to regain control of control device |33 andthe east subscriber has, therefore, gained exclusive control of theswitching circuits at station B.

At intermediate station M a portion oi the east subscribers speechcurrents are diverted into control circuit |43' causing operation of thecontrol device |44 therein. The operation of control device IM. willcause energizing currents to be immediately supplied to the winding ofrelay |4i5 causing its operation to disable the path WA by openingswitch contacts |48 therein at station M.Y

The energization of the winding of relay |45 in response to theoperation of control device Hill will be delayed after the operation ofrelay |45 for an additional interval T due to the delay circuit |41 inthe input ci relay Mt at the end of which interval the relay M willoperate to disab-le relay |45 by opening switch contacts |49.

The subscribers speech currents have been able to get through to thecontrol. circuit |63 at station M because the switch contacts 55 in thepath WA at that station, which had been previously opened in response tothe operation of control device |5| by the west subscribers speechcurrents, have now returned to their normally closed condition becausethe input of relay |52 will have been disabled by the opening of switchcontacts |53 by relay |53 in response to the operation of control device|51. The operation of relay iii?) in response to operation of controldevice |5| has been delayed for a time interval of T after relay 52operated due to the delay introduced by delay circuit |54. Thus, it isseen that the switch contacts |55 are returned to their normally closedcondition just before the east subscribers speech currents reach theabling point in path WA at station M.

The main portion of the east subscribers speech currents, the switchcontacts |31 being in their normally closed condition because of thenormally open switch contacts |33 in the input of relay |28, will betransmitted over the output of the path WA at station A to the hybridcoil H1 which will impress it on the circuit L1 over which it wil betransmitted to the west subscriber.

A portion of the east subscribers speech currents will be diverted fromthe path WA at sta-- tion A into control circuit |2| causing theoperation of control device |22 therein. The operation of the controldevice |22 will iirst cause the winding of relay |23 to be energizedwhich relay will then operate to open the normally closed switchcontacts |29 in the input of path EA at station A rendering that paththereafter inoperative to transmit the west subscribers speech currentsto the east subscriber or to the control circuits |26, |58 and |32. Theeast subscribers speech currents have now obtained exclusive control ofthe switching circuits of the whole system.

Substantiallyv di seconds after relay |23 has operated the energizingcurrents from the output of control device |22 which have been delayedin delay circuit |25 will be transmitted to the winding of relay l24causing its operation to close normally open switch contacts |30 incontrol circuit |2E rendering operative the input of relay |28. Thesystem is now biased as regards breakins in favor of the westsubscriber. The west subscriber now, on perceiving a pause by the eastsubscriber, may break in and seize control of the system away from theeast subscriber in a manner similar to that which has just beendescribed for the converse condition.

From the above description, it may be noted that the mid-point echosuppressor at station M in the system of Fig. 4 functions only duringoperationa-l changes, i. e., when the circuit goes from inactive intoactive condition, and with the circuit active, when the control of thecircuit is changed from one party to another. Therefore, the delay hasonly to be effective during this change, and may be provided for byslow-operating relays.

Although the invention has been described in connection with four-wirecircuits employing continuous two-Wire paths for transmission in eitherdirection between terminal stations, it is apparent that the principlesof the invention apply equally well to circuits employing sections oftwo-wire lines for transmittinDA and receiving at each terminal station,respectively, and employing as an intermediate link a long two-waytransmission medium, such as the air link of a two-way radio telephonecircuit or the submarine cable link of a two-way submarine cabletelephone circuit.

Although in the particular embodiments of the invention illustrated anddescribed, mechanical relays have been shown for conditioning thetransmission paths by operating movable switches to close oropen-circuit the paths, the invention is not limited to circuitsemploying the particular conditioning means shown. For example, withinthe scope o the invention the transmission paths may be effectivelydisabled by short circuits or loss networ is inserted in the paths,controlled by relays. Stationary devices having no movable elements maybe employed in place of the mechanical relay shown, for example, vacuumtube relays, such as disclosed in Crisson Patent 1,547,233 issuedNovember l, .1.926 or in the article by C. A. Beer and C. T. Evans inthe Institute of P. O. Electrical 'Engineers (London) Volume 20, 65 to72, inclusive, published in 1920.

What is claimed is:

tem for transmitting signals in opposite direc between stationscomprising means recponsiv.. to tlr gnals from one station forgivexclusive directional control of the system Dreto in preference tolater initiated signals f' ion, and means automatically operative w enid control has been. obtained by :oid s'gnals from one station to biasthe 'tvs-tcm rego, luent directional control in favor ol' later Ytedsignals from said other station.

2. The system of claim 1 and in Which said means for biasing said systemis responsive directly to the signals from said one station.

3. A system for transmitting signals in opposite directions betweenstations, comprising means responsive to the initiation of signals atone of said stations for preventing transmission of signals laterinitiated at the other station to said one station., while allowingtransmission of the signals initiated at said one station to said otherstation, and means also responsive to 'the signals initiated at said onestation for altering the circuit connections at said other station tobias the system as regards subsequent directional control oftransmission in favor of later initiated signals at said other station.

4. The system of claim 3 and in which the last-mentioned means operatesto bias the system in favor of later initiated signals from said otherstation after directional control of the system has been obtained by theearlier initiated signals from said one station.

5. In a system for transmitting signals in opposite directions betweenstations, control circuits for determining directional control oftransmission therein, means responsive to signals initiated at onestation for controlling said control circuits so as to permittransmission of said signals from said one station to another station,while preventing transmission of signals later initiated at said otherstation to said one station, and means automatically operative afterdirectional control of the system has been obtained for the signalsinitiated at said one station, to rearrange said control circuits so asto permit later initiated signals from said other station to immediatelyobtain directional control of the system at a slight pause in thetransmission of the signals from said one station.

6. In a system for transmitting signals in opposite directions betweenstations, control circuits for determining directional control oftransmission therein, means responsive to signals initiated. at onestation for controlling said control circuits so as to allowtransmission of said signals from said one station to another station,while preventing transmission of signals later initiated at said otherstation to said one station and means for rearranging said controlcircuits after directional control of the system has been obtained bythe signals initiated at said one station, to bias the system as regardssubsequent directional control thereof in favor of signals laterinitiated at said other station.

7. A four-wire signaling circuit comprising normally operative,oppositely directed one-way transmission paths connecting the terminalsof said circuit, an echo suppressor unit at or near each terminalcomprising means connected to the incoming one-way path thereat andresponsive to signals therein to disable the input of the outgoing pathat said terminal, and means connected to the outgoing path andresponsive to signals therein to disable the means for disabling saidoutgoing path, a normally disabled control circuit at each terminalwhich when operative is responsive to signals in the outgoing paththereat to disable the incoming path at the same terminal and means ateach terminal responsive to the signals in the incoming path thereat forrendering the normally disabled control circuit at the same terminaloperative a given time after the outgoing path is disabled in responseto the incoming signals.

8. A four-wire signaling circuit comprising normally operative,oppositely directed one-Way paths connecting the terminals of thecircuit, an echo suppressor unit at or near each terminal of thefour-wire circuit, each unit comprising means connected to the outgoingone-way path and responsive to outgoing signals therein to disable theoutput of the incoming one-way path, and means connected to the incomingpath and responsive to incoming signals therein to disable the means fordisabling the incoming path, a normally disabled control circuit at eachterminal which when operative is responsive to signals in the outgoingpath thereat to disable the incoming path at the same terminal, andmeans at each terminal and responsive to signals in the incoming paththereat for rendering the normally disabled control circuit at the sameterminal operative a given time after the incoming path-disabling meansat said same ter- `minal has been disabled in response to the samesignals.

9. A two-way signaling system comprising transmitting and receivingcircuits at each terminal thereof, and an echo suppressor unit at ornear each terminal comprising means responsive to signals in thereceiving circuit thereat to disable the transmitting circuit at thesame terminal, means responsive to signals in said transmitting circuitto disable the transmitting circuit disabling means, a normally disabledcontrol circuit which when operative is responsive to signals in saidtransmitting circuit to disable said receiving circuit, and meansresponsive to signals in said receiving circuit a definite time afterthey have disabled said transmitting circuit, to render said controlcircuit operative.

10. The system of claim 9 and in which said means for disabling thetransmitting circuit disabling means operates in response to signals insaid transmitting circuit substantially a time T after the initiation ofsaid signals therein, where T is the transmission time between theterminals of said system.

l1. A two-way signaling system comprising transmitting and receivingcircuits at each terminal thereof, and an echo suppressor unit at ornear each terminal of said system, each unit comprising means connectedto the transmitting circuit and responsive to signals therein to disablethe receiving circuit at a time T after the initiation of, and tomaintain it disabled an additional time interval T after cessation ofsaid signals in said transmitting circuit, where T equals thetransmission time between the terminals of said system, means connectedto the receiving circuit and responsive to signals therein to disablethe receiving circuit disabling means, a normally disabled controlcircuit which when operative is responsive to signals in saidtransmitting circuit to disable the receiving circuit and to maintain itdisabled for a deinite time after the controlling signals cease, meansresponsive to signals in the receiving circuit to render the normallydisabled control circuit operative a given time after the receivingcircuit disabling means is disabled in response to the signals, andmeans tc maintain said control circuit operative for a deiinite timeafter the signals in the receiving circuit cease.

BJORN G. BJORNSON.

